Agile View Concepts

Starting with HP-UX 11i v3, HP-UX is aware of multiple paths to devices and provides multipathing functionality automatically. Important new concepts related to this functionality are: persistent DSF, LUN hardware path, lunpath hardware path, device identifier, and agile addressing.

Agile view I/O addressing logic looks like Figure 7-2.

Figure 7-2 Agile Multiple Path I/O Stack Addressing Model

The persistent DSF represents the device, regardless of its location in the I/O configuration. With the new model, legacy DSFs and hardware paths enjoy multi-path capabilities because after device open, I/Os use every path associated with the I/O device.

The LUN hardware path is virtualized, representing all the lunpath hardware paths to a device. The lunpath hardware path is the path typically displayed in the Ignite user interface. Lunpath hardware paths do not have device special files - they are associated with a LUN hardware path and are accessed via the persistent DSF associated with a LUN hardware path. The Ignite user interface displays the lunpath hardware path so the actual device can be discerned from it; you cannot identify the physical device from looking at the LUN hardware path or the persistent DSF.

Sample agile addressing model values for the DSFs and paths are shown below.

Figure 7-3 Agile Naming Example

Identification of devices in a multiple path I/O configuration can be challenging. The I/O stack (driver) identifies devices using unique LUN IDs. Often this is a WWID value.

By identifying a device using the unique LUN ID, any of its hardware paths could be used to access it, and agile addressing is supported.

The unique LUN ID might be difficult for a user to associate with a specific physical or logical device. For example, often the device WWID is on a device label (e.g. on a sticker) or available via storage management software used to set up a virtual LUN – those values are difficult to remember and type correctly. It might be easier to select a device using one of its hardware paths. However, if you can remember part of the WWID, you can use the Ignite user interface Disk Selection dialog box Filter text box to limit displayed devices to those with WWIDs containing the text you can remember.

Agile addressing means the hardware path actually used by the system to access a device is independent of the path used to select the device. Selecting a disk via one hardware path might result in the system choosing some other, better hardware path. For example, this can happen when selecting a disk for boot, and when volume managers determine the appropriate set of paths to use for device access.

	NOTE: For HP-UX 11i v3, the one hardware path used for selection has no special significance in most Ignite-UX user interfaces. Ignite-UX will allow HP-UX system software to select the best path when a particular path is needed. For example, boot paths will be selected by system software when the boot device is selected.

A more user-friendly approach is to identify I/O devices with a device identifier. A device identifier is a human-readable device ID defined by the user. It can be written to the device and read back. Data centers may want to create some standard policy for device IDs (e.g. LAB2CAB23LUN15).

In the current implementation, the device ID can be set, checked, and read at installation. The device identifier is stored on the device, so it remains available if the disk is moved to a different system or connected to multiple systems. Not all devices support the use of a device identifier.

See the scsimgr(1M) command for more information on how to set and read a device identifier.

When identifying I/O hardware for Ignite-UX configuration files, see Table 7-3 for the format of I/O variables.

	NOTE: Your data center may use separate processes or groups to administer systems and storage. It is important to record WWID, Device ID, and other details of LUNs assigned to your systems. Access control or protection zones may be used to control the systems permitted to use a LUN; it is important to record which systems have access to LUNs.

For more details on agile view HP-UX, see the white paper The Next Generation Mass Storage Stack in the Network and Systems Management section of www.docs.hp.com, available at http://www.docs.hp.com/en/netsys.html#Storage%20Area%20Management.